For example, dry-type copying machines often employ a developing device which develops an electrostatic latent image on a photoreceptor surface into visible form with a two-component developer including carrier and toner. In such a developing device, the toner is consumed during development, while the carrier is not consumed and remains in the developing device. Therefore, when the carrier and the toner are frequently agitated in the developing device, the resin coat layer on the carrier surface is removed and the carrier adheres to the toner surface. As the carrier deteriorates, the charge of the developer gradually decreases.
To avoid such a problem, developing devices, disclosed in Japanese Publication for Examined Patent Application No. 21591/1990 and for Examined Utility Model Application No. 2596/1985, restrain the decrease in the charge of developer by little by little supplying to the developing devices the carrier separately from the toner which is consumed during development. In these devices, when the carrier is supplied, the amount of developer in the developer container increases. Then, excess developer flows into the exit formed in a wall surface of the developer container and is discharged into the developer collecting container. By repeatedly performing supplying and discharging of developer, the deteriorated developer in the developer container is replaced with newly supplied toner and carrier. It is therefore possible to maintain the charge of developer and to restrain the image quality of copies from being degraded.
In these devices, however, since the discharge of the deteriorated developer is carried out in such a manner that the deteriorated developer flows from the exit formed in the wall surface of the developer container, the ratio of the discharged deteriorated developer to the newly supplied developer can not be maintained in a desired manner. As a result, the charge of developer varies and the image quality of copies is degraded. For example, when copies are repeatedly produced from an original with high density, the consumption of toner increases and the total amount of developer in the developer container decreases. In this state, even if the carrier is supplied to the developer, the level of the developer does not reach the exit, preventing the overflow or discharge of the developer. When the deteriorated developer is designed to be automatically discharged by the overflow structure, it is hard to discharge the deteriorated developer in a desired manner. It is therefore difficult to fully compensate for the degradation of the image quality of copies caused by a decrease in the charge of developer.
On the other hand, if the carrier is supplied when a great amount of developer is held in the developer container, the newly supplied carrier may soon flow out of or be discharged from the developer container. Thus, the replacement of the deteriorated developer with the newly supplied carrier can not be carried out in a desired manner, resulting in a decrease in the replacement efficiency. In order to compensate for the insufficient replacement, excessive carrier is needed to be supplied, causing an increase in the overall consumption of the developer.
In the case of the copying machine of a compact type for personal use, if the machine is easily and frequently moved by the user, it tilts or receives impact. Consequently, a great amount of developer is automatically discharged from the exit and the image quality of copies is significantly degraded.